The first generation of women born on Earth consisted of the three sisters of Cain and Abel. Cain was born with one sister, and Abel with two. It is only in the Midrashic literature that we find a record of their existence. Since they were presumably the only females on Earth, and their brothers the only males at that time, besides Adam and Eve, the three became the brides of their brothers. For Adam and Eve, procreation was part of their punishment, but for this first generation, the ability to procreate was God’s gift, enabling the continuation of humanity. The sisters, and the three embryos shown in the lunette, symbolize fertility. Cain not only murdered his brother, but by doing so deprived Abel and his wives of any descendants. Acknowledging this great evil inflicted on her sisters, Cain’s sister/bride turns and hides her face. (http://janetshafner.com/biblical-visions/The+Sister-Brides.jpg.php)